End splitting is a phenomenon associated with the drying and shrinking processes of wood and is a common defect in dimensional lumber. End splitting occurs when the ends of a board dry at a faster rate than the interior region. This higher moisture content gradient at the ends of the beam results in differential shrinkage. When the tensile strength perpendicular to the grain is exceeded, an end split forms.
The incidence of end splits typically increases as the depth of the wood increases. For example, dimensional 2.times.10 and 2.times.12 lumber have a higher occurrence of end splits when compared to 2.times.4 and 2.times.6 lumber. End splits also are more common on the higher-density species of wood such as Douglas Fir, and less common on lower-density species such as Spruce and Pine. On average, about 20% of a quantity of dimensional lumber will have end splits.
The moisture content of lumber varies and is dependent on its exposure to weather. End splits can occur in a piece of lumber at any time, i.e., during the period from initial milling up until the time the piece reaches moisture content equilibrium in its final structure. An end split can start small and become longer, propagating toward the middle of the piece of lumber.
As end splits are a common defect in lumber, specified lengths of end splits are permitted by lumber grading rules. Typically, the higher the grade of lumber, the smaller the allowable end split. Indeed, the grade of a piece of lumber can be governed by an end split. For example, a piece of lumber which is Select Structural grade in all respects except for the permissible length of an end split, could be downgraded to a No. 3 grade.
Plates have been applied to composite wood beams to increase shear capacity. For example, Knowles U.S. Pat. No. 4,637,194 describes applying shear stress plates to opposite faces of a web of a wooden I-beam near the ends of the beam. The plates may be formed of a plywood and secured to the web by means of nails, staples, adhesive, or a combination thereof. The shear stress plates are said to increase the thickness of the web only in the shear-field or area needed. Alternately, a stiff metal plate may be used, similar to that used in the truss industry. The metal plates are said to reinforce the ends of the web in a manner similar to thickening the ends of the web in the area or field of maximum shear stress.
The shear plates disclosed by Knowles function by the classical formula for calculating maximum longitudinal shear capacity of wood beam: EQU V=2/3Af.sub.v
V=maximum shear force PA1 A=cross-sectional area of wood beam PA1 f.sub.v =allowable unit shear stress of wood
From this classical formula, the maximum shear force in a wood beam is directly proportional to the cross-sectional area of the beam. Based on the uniform shear stress distribution shown in FIG. 14 of Knowles, it is apparent that no consideration was given to the end split condition. The classical formula described by Knowles is not valid for end-split lumber, which does not have a uniform shear stress distribution.
The use of metal plates also is disclosed by Birckhead U.S. Pat. No. 4,442,649, wherein a fabricated wooden beam has at least two longitudinal wooden members spaced parallel and apart by a plurality of wooden blocks spaced between and along the length of the longitudinal members. A plurality of truss plates, each installed across one side of a block and spanning between each longitudinal member, are said to provide structural integrity. The truss plates may be installed in pairs with one across either face of each block at an acute angle, i.e., a 45.degree. angle, to the longitudinal axis of the beam. Narrow truss plates may be provided at each end, tieing the ends of the longitudinal members and the end block more securely together.
Although Knowles and Birckhead describe applying shear plates to composite wood beams to increase shear capacity and/or assemble the composite wood beams, neither reference addresses the use of lumber with end splits or the problem of inhibiting end splits from propagating in dimensional lumber to upgrade or avoid downgrading of the lumber.